Tunable laser sources and OSAs ease component characterization

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Laurent Begin and Dave Kritler

As new technologies are developed for next-generation optical transport equipment, cost-reduction efforts become essential. The authors explore the different technical directions test and measurement companies are taking to support these challenges.

Tunable laser sources and optical spectrum analyzers (OSAs) have become indispensable tools for test engineers involved in WDM. As system designers work to expand the transmission bandwidth, they reduce channel spacing (to 50 GHz and lower) and extend the spectral transmission window (to the L- and S-bands). As channels get closer, resolution and accuracy become decisive factors.

At the instrument level, five main parameters specify a tunable laser souce: tuning range vs. power, wavelength resolution (limited by laser line width), wavelength accuracy, tuning speed, and signal-to-source spontaneous emission (SSE) ratio. When testing WDM narrowband components it is necessary to achieve accurate high-resolution measurements. Users also require speed and the capability to cover various telecom spectral bands with a limited number of sources. Whether for optical components characterization or for general use, the best combination of features has been provided by external-cavity-based tunable laser sources (see table, p. 60).

In an external-cavity laser, the tuning is achieved by mechanically modifying the laser layout (see Fig. 1). In our example, the rotation of the dihedral reflector combined with the grating selects the operating wavelength. The Littman-Metcalf configuration of this laser is theoretically mode-hop free, allowing continuous tuning of the emitted wavelength (this is the only way to achieve continuous scans when testing components).

As long as the reflector executes the proper rotation, the laser operates on the same mode of the cavity. However, tolerance is tight and it is difficult to obtain the proper alignment continuously, especially across wide tuning ranges. Active control of the laser optical alignment to ensure that it is always operating in its optimum configuration extends the laser performance and enhances its reliability.

REDUCING TESTING TIMES
A tunable laser source that combines this concept with feedback control features mode-hop-free tuning because the optical elements of the laser are aligned in real time based on feedback signals. Moreover, when properly designed, this method allows fast sweeping operation (100 nm/s) while maintaining performance. Together with a solution to accurately measure the wavelength as fast as it is tuned, this is a very effective tool for component testing that drastically reduces test times.

The same approach can be used to extend the laser tuning range. By adjusting optical-element positions according to the emitted wavelength, it is possible to compensate for chromatic effects that limit the range of accessible wavelength. Using these principles, we have achieved more than 190-nm mode-hop-free swept scans at more than 0 dBm output power and at speeds up to 100 nm/s (see Fig. 2).

High-accuracy wavelength measurements are available through the use of Michelson-based wavelength meters. These devices achieve picometer accuracy but have two drawbacks for our application: they are packaged in an external box and are too slow for a fast-sweeping tunable laser source. Adapting the Michelson principle, it is possible to integrate a small form-factor wavelength coder into the tunable laser source itself. Combined with a feedback loop, this approach delivers high wavelength accuracy and enhances the long-term stability that is mandatory for accurate process monitoring (when growing thin-film filters, for instance). We have achieved a better than ±5 pm accuracy with picometer stability.

Once you have achieved wide tuning, resolution, speed, and accuracy, how can you improve the signal-to-SSE ratio? Different solutions involve the use of a tracking filter or an OSA to cut the noise. This method leads to higher dynamic but is limited because output power is significantly reduced and filters are not sufficiently selective.

A more efficient approach is to build an intrinsically noise-free laser. The noise comes from the amplified spontaneous emission (ASE) of the diode, which is emitted in the same direction as the signal and is coupled with the signal in the laser output fiber.

A new configuration to address this issue has a design in which the signal is picked after the grating before going back into the diode (see Fig. 3). Combining a periscope beamsplitter with the grating and the dihedral reflector forms a Sagnac interferometer similar to those used in optical gyroscopes. This design allows only the light coming from the grating to be coupled into the output fiber. The broadband ASE noise is filtered out after being diffracted by the grating. With this configuration, a tunable laser source can achieve a signal-to-SSE ratio greater than 90 dB.

OPTICAL SPECTRUM ANALYZERS
In characterizing WDM transmission channels with an OSA, high optical resolution is one part of the solution. It has to come with high dynamic to ensure quality OSNR measurement, especially as channel spacing decreases (to 50 GHz and below). An efficient way to achieve this is through spatial filtering.

High-performance OSAs rely upon a diffraction grating that disperses light based on its wavelength. The narrowest spectrum an OSA can accurately measure depends on its optical resolution. A WDM transmission channel has a subpicometer-wide spectrum (with no modulation) and is below any OSA resolution. When analyzing a laser line, an OSA will not display the channel' s spectrum, instead showing a broader function characteristic of its performance. The width of this transfer function at half its maximum defines the OSA optical resolution, typically 10 to 60 pm.

The OSA gives valid information on the channel power and wavelength, but anything related to shape does not relate to the channel itself. So when measuring OSNR, it is possible to accurately evaluate the signal, but the "legs" of the OSA transfer function may conceal the noise, especially if the channels are closely spaced and their associated transfer functions start to overlap. The same is true when characterizing modulated WDM transmission channels and their multiple harmonics.

A work-around solution is to increase the OSA optical resolution, but this only takes care of the 3-dB width. A more efficient solution is to trim the OSA transfer function. The root of the limitation to the OSA dynamic comes from stray light that ends up at the detector together with the signal (this is mostly due to light scattered by the grating). This causes the noise that broadens the transfer function.

An effective solution to increase dynamic is to filter out this noise by introducing a slit in the optical path of the OSA (see Fig. 4). The slit acts as a spatial filter and is positioned to limit the borders of the optical path followed by the signal and to block any light following a different path. The light scattered by the grating goes in every direction and will be spatially filtered. Using this configuration we have characterized 12.5-GHz ultradense-WDM channels with a dynamic of more than 40 dB (see Fig. 5).

TEST SOLUTIONS
After selecting adequate instruments for your test application, the engineering team must work to get them to function together. Fortunately, test and measurement companies offer a number of turnkey test solutions for generic applications.

When characterizing passive optical components, the solution of choice for final testing combines a sweeping tunable laser with multiple broadband optical-power meters. For in-process testing, engineers still appreciate the combination of an OSA with a broadband source because of its lower cost. This method provides speed, sufficient resolution, accuracy, and dynamic, and most of the time only a small number of the device ports must be characterized.

An alternative is to combine a sweeping tunable laser with a limited number of power meters in a single, full-featured instrument. It provides all the functionalities of an OSA with higher resolution and accuracy, a built-in source, and a few detectors. Using a sweeping ASE-free tunable laser source, we have measured in seconds DWDM filters with 3 pm of accuracy and 75 dB of dynamic. This type of performance is also adequate for final testing of devices having small numbers of ports, such as fiber Bragg gratings and thin-film filters.

INTEGRATION WITH MANUFACTURING
As markets, products, and processes mature, manufacturing turns from matters of production to matters of reduction. Test-equipment vendors facilitate this process by improving critical measurement technologies and merging them into task-targeted, higher-performance, low-cost test systems that deliver measurable cost savings and production capacity increases.

Despite advances in test instrument productivity, however, the persistent problem of fiber handling still must be addressed to minimize passive-component production costs. Fiber end handling during the testing process is a major problem for fiber and cable manufacturers, but it is already a bigger one for many component manufacturers struggling with multiport devices. The latest component test systems need to include improved fiber-handling methods to simplify and speed component preparation and coupling of the pigtail to the test system.

Ultimately, automated fiber-handling systems may interface with automated material handling to deliver even higher production output and lower costs. Many passive-component manufacturers are taking this path, and test-solution providers must continue to lead the progress if low-cost, high-volume components are to prevail in the same way. Especially for manufacturing applications, test instruments must be integrated into complete solutions. The extent to which component-rich optical communication systems are deployed depends on this transition.

Laurent Begin is product marketing manager and David Kritler is marketing manager for NetTest Optical Group, 9405 SW Gemini Dr., Beaverton, OR 97008. Laurent Begin can be reached at laurent.begin@nettest.com.

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